First off , here's a couple of facts you can't argue with in the great Horse Blanket debate!
A horse blanket can get wet and soaked through very quickly—imagine yourself on a cold windy day wearing a wet jacket and no one around to get you a warm dry replacement—your horse wearing a blanket that has gotten wet will chill him super fast and could cause health issues (and even the waterproofing on a waterproof blanket wears off quickly so check it often).
Together with his fluffy long hair, the biggest way your horse stays warm is by eating hay. That keeps his internal furnace working. So be sure to provide enough hay and enough fresh not frozen water (no, horses cannot live on snow to provide their water intake) especially on really cold winter days and nights the best thing is free choice hay and water. As long as you take care of that forage and you provide a place for the horse to get out of the wind, his natural built-in-blanketing system will work.
When your horse grows his winter coat the hair has the ability to loft and lower depending on the temperature. According to the latest research done at the Colorado State University, one of the top veterinary schools, the horse's coat can lower and loft to 17 different levels. It's like they have a built in blanketing system with their own hair coat that acts like 17 different weights of thermal blankets being put on and taken off according to the weather. When you put a blanket on a horse that has his own winter coat of long hair you squash those lofts and layers and you may actually be making him colder.
So you can see that is is not a black and white answer and even that in the majority of cases NOT blanketing is the best decision for your horse.
So when should you blanket your horse?
Keep in mind that if you are blanketing your horse you have to take it off every day and groom him –they get itchy! Keep the blanket dry in all kinds of weather and be prepared to purchase a few different weights of blankets and be there to switch them around depending upon the temperature. If your horse is sweaty from a workout make sure you cool him out properly first with a cooler before blanketing him.
Nature knows best so be sure to work with nature and not against it!